Adapt numerical base transformers to narwhals-returning check_X (#1022)

Author: solegalli

feature_engine/_base_transformers/base_numerical.py

@@ -61,7 +61,7 @@ def _fit_setup(self, X: IntoDataFrame):
         """
 
         # check input dataframe
-        X = check_X(X)
+        check_X(X)
 
         # find or check for numerical variables
         if self.variables is None:
@@ -114,7 +114,7 @@ def _check_transform_input_and_state(self, X: IntoDataFrame) -> IntoDataFrame:
         check_is_fitted(self)
 
         # check that input is a dataframe
-        X = check_X(X)
+        check_X(X)
 
         # Check if input data contains same number of columns as dataframe used to fit.
         _check_X_matches_training_df(X, self.n_features_in_)

feature_engine/_base_transformers/mixins.py

@@ -96,7 +96,7 @@ def _fit_from_dict(
             The variables in the dictionary.
         """
         # check input dataframe
-        X = check_X(X)
+        check_X(X)
 
         # find or check for numerical variables
         variables = list(user_dict_.keys())